The instant invention relates generally to an artificial prosthestic foot, and more particularly, to a biomechanical ankle type prothesis.
The biomechanics of the foot and ankle represent a complex set of various forces and motions. Those members cannot be each considered as a separate entity, but rather as an integral part of the biomechanics of the entire lower limb. The human body requires a flexible foot in order to adapt to its external environment, which may be flat, uneven, or sloping. Without this freedom of motion a person would be restricted in every day activity.
One basic type of motion sequence that occurs in the ankle joint is dorsiflexion followed by plantar flexion, or vice versa. This sequence about the angle axis of motions is basically perpendicular to the line of forward progression. The ankle joint undergoes plantar flexion which is defined as downward flexion, at the time of initial floor contact (heel strike), which continues until the onset of midstance phase or throughout the first 15% of the walking cycle, and then progressive dorsiflexion which is defined as opposite or upward flexion, occurs from the time of heel-off until the 40% point of the cycle, when again, plantar flexion begins. Thus, during the swing phase, dorsiflexion of the ankle joint takes place until the time of heel strike, when plantar flexion again begins.
Another basic type of motion sequence which occurs in the ankle/foot structure is inversion and eversion. This motion takes place about the subtalar axis, which extends essentially parallel to the longitudinal axis of the foot or perpendicular to the ankle axis. Eversion in simple is moving the sole of the foot outward at the ankle joint. Inversion is moving the sole of the foot inward at the ankle joint. The axis of rotations for these motions is essentially parallel to the line of progression. These maneuvers of the ankle/foot complex allows a person to walk on unlevel surfaces and sloping surfaces without difficulty.
In executing the above motions, the muscles within the lower limb play a vital role in the walking function. By virtue of muscles, controlled and selective movement in the skeletal system is made possible. Muscle movement is actually muscle contraction. Muscle contracture is defined as a condition in which a muscle shortens its length. Therefore, with muscle action on both sides of an axis of a skeletal member, a constant and reccurring control of motion is made possible.
In today's prosthetic field there are various alternatives, many of which are poor designs, to produce the complex actions of the foot/ankle. One such prosthetic device is known as the solid ankle cushion heel foot. That type of prosthetic assembly does not allow a patient to have controlled motion of the foot because it is fabricated of solid material. The motion is predetermined and prefabricated at the factory. Another type of prosthetic device is that known as a single axis foot. That design has only a single axis and can only be ued in above knee amputations because of its size. Also, the subtalar joint action is lacking in both of those prosthetic devices. Among the distinct advantages and features of the present biomechanical ankle is that it not only offers controlled subtalar motion, both plantar and dorsiflexion, but additionally is small enough to be used in a below the knee amputation.
The present biomechanical ankle allows duplication of all types of normal ankle/foot motions, which is, both dorsal and plantar flexion, as well as inversion and eversion. To duplicate the dorsal and plantar flexion of the ankle mortice, controlled acentric and concentric contractures of the ankle/foot must be produced. In the normal ankle mortice these motions occur in a transverse axis and are made possible by a series of related muscles and ligaments. Thus, in prosthetic ankles, the objective is to produce this same action without the use of voluntary muscles. The present biomechanical ankle has the same transverse or ankle axis of the normal ankle mortice. To duplicate the muscle movement of the ankle/foot, which occurs in concention and exertion action, the present device incorporates a control spring which functions in the manner described hereinafter.
As brought out above, the inversion and eversion motions of the normal ankle mortice occur at the subtalar joint, which is the ankle bone structure about the subtalar axis which as mentioned above extends essentially parallel lengthwise to the foot. The subtalar actions of the present biomechanical ankle are realized by virtue of the above referred to spring in combination with the various positioned hinges embodied in its design as described in detail hereinafter.
The primary objective of the instant invention is to provide an artificial prosthesis which meets all the criteria of a prosthetic ankle. Such criteria includes durability; light weight; the capability to support tremendous weight and torque forces, as well as compress to move dynamically similar to a normal foot; and small enough for cosmetic purposes. The most important net objective realized by way of this invention is that a patient can develop a normal gait with the least amount of energy expenditure.